The invention generally relates to a device for machining objects using laser beams.
Laser machining units today frequently use two or even more different laser beams, which are directed independently of each other onto the object to be machined. It is not always necessary to have a number of different laser light sources to generate the different laser beams. The laser beam emitted by a single laser suffices, for example, when a beam splitter is used to split it into a number of sub-beams, which can then be directed onto the object to be machined.
In many cases different laser light wavelengths are used, in particular to be able to process different materials with a high level of precision. Two or more laser light sources are generally used to generate two or more different laser light wavelengths. Alternatively a single laser can also be used, with the beam emitted by this laser being split into at least two sub-beams using beam splitters and with the wavelength of at least one of the two beams being changed by way of a non-linear optical effect (in particular what is known as frequency multiplication). In this way a single laser light source can be used to generate two different sub-beams just as when two laser light sources are used, with the spectral distribution of the one sub-beam being different compared with the spectral distribution of the other sub-beam.
Simultaneous use of a short-wave laser beam and a long-wave laser beam is required in particular for the laser structuring and laser drilling of multilayer circuit boards. Here, for example, the short-wave laser light, which also strikes the object to be machined in the shortest possible laser pulses, can be used to strip thin metal layers, which have formed both on the surface and inside the multilayer circuit boards and which separate different non-conducting layers. The longer-wave laser light is used to strip these non-conducting layers precisely. In this way, for example, holes can be created with a diameter of 20 xcexcm or less, so that subsequent coating of a laser-drilled hole with metal will allow specific metal intermediate layers to be connected together in an electrically conductive fashion in an extremely small space.
Laser machining with two different wavelengths is currently carried out generally by using adjacent deflection units to direct two laser beams accurately onto the object to be machined. Each of the deflection units generally has two rotatable mirrors, so that the laser beam striking the object to be machined can be positioned in an x-y plane. The arrangement of the two deflection units has the disadvantage that the two laser beams, in so far as they are to be directed onto a common sub-area of the object to be machined, strike the machining surface at different angles. The different angles of impact of the two laser beams are particularly disadvantageous when laser-drilling small holes, as the different angles mean that the resulting diameter of the drilled hole becomes bigger as the hole gets deeper, particularly at the top edge of the hole. Also, when the angle differences are large, holes are drilled obliquely into the object to be machined. In the case of through holes, for example, this results in the hole on one side of the object having a different position than on the opposite side, so the precision of the drilled hole deteriorates significantly.
An object of an embodiment of the invention is therefore to create a device for machining objects using laser beams, which allows the essentially perpendicular machining of objects using two different laser beams.
An object my be achieved by a device for machining objects using laser beams. An embodiment of the invention is based on the knowledge that the best possible perpendicular machining of objects can be achieved using two different laser beams, if the beam paths of the two laser beams are at least approximately merged using a partially reflective optical element. The device according to an embodiment of the invention has the advantage that the two deflection units can in principle be located at any distance from each other, without this resulting in any deterioration of the beam control of the two laser beams directed onto the object due to significantly different angles of impact.
The device according to an embodiment of the invention can in particular be used advantageously for laser drilling, if the two laser beams are superimposed at the partially reflective optical element such that their beam paths strike the object to be machined in an essentially coaxial fashion. In this way, with precise control of the output of the two laser beams, both through holes and also what are known as blind holes can be drilled quickly and with a high level of precision.
The device according to an embodiment of the invention can however also be used advantageously for laser drilling, if the two laser beams are directed parallel with a specific displacement onto the hole to be machined. In this case two different holes can be drilled at the same time.
The most important advantage of the device according to an embodiment of the invention is that two separate laser beams can be directed completely independently of each other onto one and the same machining field to machine an object. This opens up a large number of possibilities for machining different types of objects precisely and quickly with a single machining unit.
The use of flat field lenses has an advantage that the two laser beams can be directed onto the object to be machined within a large machining area without the beam quality changing due to different focal widths. Such a change in focal widths generally occurs in standard spherical lenses, in which the focal area, i.e. the area in which the laser beam is focused for different angles of incidence of the laser beam, is on a spherical surface. Flat field lenses, which are also referred to as F-theta lenses, unlike standard spherical lenses, are characterized by the fact that the focal area is in a plane largely independent of the angle of incidence of the light beam striking the flat field lens. The use of flat field lenses therefore allows precise focusing of the machining laser beams within a large machining area, so that large objects can also be machined without interim displacement and therefore without interruption.
An embodiment of the invention in which the two deflection units are arranged perpendicular to each other, has an advantage that when the two deflection units are in the zero position the two laser beams strike the partially reflective optical element perpendicular to each other. This simplifies the structure and in particular the optical adjustment of a corresponding laser machining device, as when the two deflection units are in the zero position, the laser beams can exclusively be guided perpendicular or parallel to the machining surface.
In one embodiment, the two laser beams have different wavelengths and a dichroic mirror is used as the partially reflective element. The use of a dichroic mirror has the advantage compared with the use of conventional semi-permeable mirrors that, subject to appropriate spectral reflection and transmission characteristics on the part of the dichroic mirror, the output of the two different laser beams can be used to machine the object to be machined without major loss. Contrary to this, if a conventional semi-permeable mirror is used, both unwanted reflection of the transmitted laser beam and unwanted transmission of the reflected laser beam would occur. The intensities of these unwanted laser beams would result in a loss of output, which would have an adverse effect on the thermal stabilization of a laser machining unit, if the beams were merged using a conventional semi-permeable mirror. The use of a dichroic mirror tailored to the wavelengths of the two laser beams therefore facilitates thermal stabilization of the laser machining unit by reducing loss of output and as a result contributes to a consistently high level of machining accuracy over time.
In one embodiment, a first laser is used to create the first laser bean and a second laser to create the second laser beam.
As an alternative to using two laser light sources, the two laser beams can also be generated by a single laser, with the spectral distribution of at least one of the two laser beams being changed by what is known as frequency conversion. Frequency conversion, in which the frequency of the primary laser beam is increased, is for example illustrated by frequency multiplication within an optically non-linear crystal. Frequency conversion can however also be used to reduce the frequency of the primary laser light. This can be achieved, for example, by frequency mixing, in which two different light beams with different spectral distributions are spatially superimposed in an optically non-linear crystal, so that both the total frequency and in particular the differential frequency are generated between the frequencies of the two mixed light beams.
Efficient beam merging without loss of output due to unwanted reflected or unwanted transmitted laser beams can also be achieved with two laser beams of the same wavelength. For this, a polarization-dependent mirror may be used as the partially reflective optical element. Efficient use of a polarization-dependent reflector requires the polarization directions of the two laser beams to be different, ideally perpendicular to each other. For example, what is known as a Nichol prism or in principle any other optically active material with different refractive indices for different polarization directions can be used as a polarization-dependent mirror.
The polarization direction of light beams striking the polarization-dependent mirror may be influenced by an optically active crystal. For example, a light beam initially polarized linearly in a specific direction can be rotated through 90xc2x0 using what is referred to as a xcex/4 plate. However other materials, the optical activity of which is based on the magneto-optical effect (Faraday effect) or the electro-optical Kerr or Pockel""s effect, can also be used as the elements to rotate polarization. It should also be pointed out that polarizing optical elements, such as, for example, a polarizer film or even a Nichol prism can be used to generate a polarized light beam from an initially unpolarized light beam.